Many radio communication devices must support signal transmissions with different transmit power requirements. For example, a radio may have dual mode operation in which one mode requires signal transmission by frequency modulation (FM), and another mode requires signal transmissions by amplitude modulation (AM), or variations thereof, such as quadrature amplitude modulation (QAM). In another example, the transmitter must be able to support transmissions of signals using a particular modulation scheme at various output power levels. Generally, it is desirable to have the transmitters of these multi-mode radios to be capable of operating efficiently, regardless of operating mode.
A typical radio transmitter employs a radio frequency (RF) amplifier to amplify outbound signals for radiation by an antenna. The efficiency of the RF amplifier is a significant contributor to the overall efficiency of the transmitter. RF power amplifiers are generally designed to provide maximum efficiency at peak output power. When the output power level is adjusted to values below the peak output power, such as by varying the input drive level to the power amplifier, a significant efficiency reduction occurs. This is often the case regardless of the class of amplifier involved. As efficiency considerations are of primary concern in many of today's RF communications applications, the provision of an efficient power amplification system for a radio transmitter has received much attention in the art.
FIG. 1 is a block diagram of prior art amplification circuitry 10 that attempts to address the problem of transmitting at different output levels while maintaining high efficiency transmissions. In the amplification circuitry 10 depicted, two separate amplifiers 13, 23 are coupled to an input source 12, and an RF switch, comprising pin diodes 15, 25, is used to select the amplifier 13, 23 best suited for a given function. Direct current (DC) is used as a bias to enable a selected amplifier 13, 23 to present an output 32 for routing to an antenna (not shown). In operation, the pin diodes are biased "on" to select the first amplifier and to establish an RF path from the first amplifier to the antenna. The diodes are reversed biased "off" to select the second amplifier, and to form an RF path from the second amplifier to the antenna. Reverse biasing is necessary on the diodes to reduce the RF signal distortion, as the "off" diodes are susceptible to large RF signal levels. The use of separate amplifiers increases the overall cost, size, and weight of the radio transmitter.
Another approach which can be found in the prior art includes the use of cascaded amplification stages, one or more of which may be switched out, such as by using a diode switch, to effect different power output levels. An example such a variable power amplifier is described by U.S. Pat. No. 5,276,917, issued on Jan. 4, 1994, to Vanhanen et al. for a Transmitter Switch-On In A Dual-Mode Mobile Phone. Yet another approach selects between impedance networks which are selectively attached to an amplifier output in order to vary the overall output signal level. An example of this approach is described by U.S. Pat. No. 5,361,403, issued on Nov. 1, 1994, to Dent for an AM-FM Transmitter Power Amplifier. Generally, the various RF signal paths are selected using RF switching employing diodes and the like.
The use of separate amplifiers to achieve a multiple output power level requirement adds unnecessary costs and size to a radio transmitter. Many of the other approaches to multiple amplification output power level at high efficiency depend on the use of RF switching, which may be generally susceptible to leakage at a high RF level, resulting in potential distortion in the output signal. It is desirable to have a radio transmitter with power amplification which do not suffer from the above-mentioned problems. Therefore, a new approach to multi-mode power amplifier design is desired in the art.